$7\mathrm{.}30$ The measured dissolved oxygen concentration using a DO meter of a river is $6$ mg$/$L$.$ The oxygen deficit is $2$ mg$/$L at the same location. What is the saturation concentration of dissolved oxygen in: $($a$)$ mg$/$L$,($b$)$ ppm$,($c$)$ ppb$,$ and $($d$)$ moles$/$L$?$